1. Field of the Invention
The present invention relates to a high-reactivity liquid polyurethane composition which contains uretdione groups and cures at a low baking temperature, to a process for preparing the polyurethane composition and to its use for producing plastics, particularly surface coatings and adhesives.
2. Description of the Related Art
Externally or internally blocked polyisocyanates are valuable crosslinkers for thermally crosslinkable polyurethane (PU) coating and adhesive compositions.
For instance, DE-A 27 35 497 describes PU coatings having an outstanding weathering stability and thermal stability. The crosslinkers whose preparation is described in DE-A 27 12 931 are composed of ε-caprolactam-blocked isophorone diisocyanate containing isocyanurate groups. Also known are polyisocyanates containing urethane, biuret or urea groups, whose isocyanate groups are likewise blocked.
The drawback of these externally blocked systems lies in the elimination of the blocking agent during the thermal crosslinking reaction. Since the blocking agent may thus be emitted to the environment it is necessary on ecological and workplace safety grounds to take particular measures to clean the outgoing air and to recover the blocking agent. The crosslinkers, moreover, are of low reactivity. Curing temperatures above 170° C. are required.
DE-A 30 30 539 and DE-A 30 30 572 describe processes for preparing polyaddition compounds which contain uretdione groups and whose terminal isocyanate groups are irreversibly blocked with monoalcohols or monoamines. Particular drawbacks are the chain-terminating constituents of the crosslinkers, which lead to low network densities in the PU coatings and hence to moderate solvent resistances.
Hydroxyl-terminated polyaddition compounds containing uretdione groups are subject matter of EP 0 669 353. Because of their functionality of two, they exhibit improved resistance to solvents. Compositions based on these polyisocyanates containing uretdione groups share the feature that, during the curing reaction, they do not emit any volatile compounds. At not less than 180° C., however, the baking temperatures are at a high level.
The use of amidines as catalysts in PU coating compositions is described in
EP 0 803 524. Although these catalysts do lead to a reduction in the curing temperature they exhibit considerable yellowing, which is generally unwanted in the coatings sector. The cause of this yellowing is presumed to be the reactive nitrogen atoms in the amidines. They are able to react with atmospheric oxygen to form N oxides, which are responsible for the discoloration.
EP 0 803 524 also mentions other catalysts which have been used to date for this purpose, but without indicating any particular effect on the curing temperature. Such catalysts include the organometallic catalysts known from polyurethane chemistry, such as dibutyltin dilaurate (DBTL), or tertiary amines, such as 1,4 diazabicyclo[2.2.2]octane (DABCO), for example.